Process for tanning leather



2,970,031 Ice Patented Jan. 31, 1961 PROCESS FOR TANNING LEATHER Daniel E. Nagy, Stamford, Conn., assignor to American Cyanamid Company, New York, N.Y., a corporation of Maine No Drawing. Filed Aug. 31, 1955, Ser. No. 531,830

6 Claims. (Cl. 8-94.27)

; and thereafter treating the resulting stock with an alkali metal cyanate.

One of the objects of the present invention is to treat pickled stock with a tanning material and an alkali metal cyanate in order to tan leather. A further object of the present invention is to tan leather by a process which is more easy to control than the prior art processes and, at the same time, to produce leathers of uniformly desirable qualities. These and other objects of the present invention will be discussed in greater detail hereinbelow.

In the tanning of leather, hides are generally washed and treated with lime liquor in order to remove hair and other extraneous matter. The hides are then washed and bated such as with an enzyme. The bated hides are washed and then pickled by saturating the skins with a solution of sodium chloride and suflicient sulfuric acid to reduce the pH to about 2.5-3. This product is commonly referred to as the pickled stock. The pickled stock may then be treated with a metallic tanning material such as hydrated chromium sulfate dissolved in water. This tanning step can be accomplished by a drum or a paddling operation. In the drumming operation, the hides are introduced into a drum-like vat containing the aqueous solution of the hydrated chromium sulfate and the drum is rotated in order to soak the hides thoroughly. In the comparable paddling operation, large paddles are used to circulate the hides through the aqueous solution of the hydrated chromium sulfate in order to get the chromium into the skins. In the prior art, it has been conventional to accomplish a setting step, which amounts to the neutralization, at least in part, of the acidic material in the hides. This has been accomplished by adding, in relatively small increments over a period of time, such alkaline salts as sodium bicarbonate, sodium borate, sodium acetate and the like. The use of such a setting step required the constant diligence and attention of the operator to insure that the setting took place neither too rapidly nor too slowly. This type of operation frequently resulted in non-uniform products as the measure of neutralization on any given hide would vary from section to section, not only on the individual layers of the hide but also in cross-section. The hides thus treated are then generally washed and a sample may be removed for g the purpose of testing for shrinking. The remaining sections of hides thus treated are then subjected to the oiling step in which the washed hides are introduced into a rotatable drum comparable, if not identical, to the one previously discussed, containing an emulsion of oil or mixtures of oil such as sulfonated oils and raw oils. The drum is then rotated for a desired period of time depending on the properties required in the treated leather. The oil treated hides may then be washed and dyed and then dried or they may be washed and treated additionally with a vegetable tannin, if desired.

According to my invention, the metal tanned pickled stock is set or neutralized by the addition thereto of an aqueous dispersion of an alkali metal cyanate. The alkali metal cyanate is permitted to react on the metallic tanning material in the pickled stock for a desired period, whereupon the so treated leather is washed and the rest of the conventional procedure is then carried out. It should be noted that the alkali metal cyanate need not be introduced in small increments but rather it can be introduced in bulk fashion and since this is a more expeditious manner of handling the process, it is preferred. This expeditiousness not only saves time in the production of the tanned leathers but also saves in labor costs and yields more uniform leather products.

The amount of alkali metal cyanate, which will be used will vary over a wide range depending on the type of leather desired. One skilled in the art, having learned of my invention, will know that he may use that amount of alkali metal cyanate which is necessary to raise the pH of the tanned hide to the desired level. Ordinarily, the pH will not be raised much above 5 to 5.5, although for special purposes, pHs of 6 or higher may be used. The amount of alkali metal cyanate used can, as a consequence, be varied, for instance, between about 0.1% and 10% by weight based on the weight of the pickled stock.

The alkali metal cyanate used may be potassium cyanate, sodium cyanate and the like. Obviously, these alkali metal cyanates may be used either singly or in combination with one another.

Among the tanning agents which may be used in the practice of the process of the present invention are the chrome tanning materials, aluminum tanning materials, ferric tanning materials, zirconium tanning materials and the like. Specific illustrations of these metallic tanning agents are chromium sulfate, chromium chloride, aluminum sulfate, aluminum chloride, ferric sulfate, ferric chloride, zirconium sulfate, zirconium chloride and the like. Quite obviously, these tanning materials may be used either singly or in combination with one another.

If, in the practice of the process of the present invention, it is desired to neutralize the metal tanned pickled stock with an alkali metal cyanate followed by a further pH adjustment with any of the conventional neutralizing agents mentioned hereinabove, the same may be accomplished.

In order that the present invention may be more completely understood, the following examples are set forth in which all parts are parts by weight unless otherwise stated. It should be remembered that these examples are set forth primarily for the purpose of illustration and any specific enumeration of detail contained therein should not be interpreted as a limitation on the case except as is indicated in the appended claims.

Example 1 224 parts of pickled calf skin are introduced into a rotatable drum into which there is introduced 167 parts of water having dissolved therein 22.4 parts of Tanolin R (hydrated chromium sulfate) and 8.35 parts of sodium chloride. The skins are tumbled therein in the closed drum for about 30 minutes. 2.16 parts of potassium cyanate dissolved in parts of water are added to the chromium treated skin in one portion and tumbled in the closed rotatable drum for about 40 minutes. The final pH of the solution in which the skins were tumbled was 3.95 and the liquid was clear. The skin is then removed, washed in water for 10 minutes at 35 C. and tumbled in 200 parts of 50 C. water containing 11.2 parts of Tanoyl 1230 R (a sulfonated oil) for 45 minutes. The sample is then removed and subjected to a conventional shrink test. The shrink temperature of the leather thus treated was 845 C. The leather produced was uniformly tanned and was very light blue in color.

Example 2 place of the 2.16 parts of potassium cyanate, there is used 4.48 parts of sodium bicarbonate. The final pH was 4.1 and the water was cloudy; The shrink temperature was 69 The leather waslnotfunifo'rmly't'anned and w'as a dark bluish-green color. It quite" obvious: that the leather produced'according to th'epr'oc'ess of Example l is far superior tothe leather produced according to Example 2, not only because of th'e'higher' shrink' temperature'but alsobecause of uniform; light colored ta'n'n'age;

1', A process for tanning'leather'whi chcompris'es'first soaking pickled hides in a water solution of a mineral tanning agent and then impregnating the resulting hides containing said mineral tanning. agent with an aqueous alkali metal cyanate solution until a c u'antityl of alkali metal cyanate has been absorbed sufii'c'ie'nt to set the mineral tanning agent therein" and convert the hides into leather.

2. A process according to claim 1 wherein the alkali metal cyanate is potassium cyanate.

-3. A process for tanning leather which comprises first soaliin'g pickled hides in a water solution of a watersoluble chromium salt tanningagent and thenimpregnating the resulting hides containing the chromium salt with an aqueous alkali metal cyanate solution until a quantity of alkali metal cyanate has been absorbed sufiicient to set the chromium salt tanning agent therein and convert the hides into leather.

4. A process according to claim 3 wherein the alkali metal cyanateis potassium cyanate.

2,684,282 Wormell July 20, 1954 FOREIGN PATENTS 721;,14l Germany Tune 1, 1942 888591 Germany Sept. 3,.1953

OTHER REFERENCES ILILSLTLCL, July'19'3'4, vol, 18, pp. 388-392; 

1. A PROCESS FOR TANNING LEATHER WHICH COMPRISES FIRST SOAKING PICKLED HIDES IN A WATER SOLUTION OF A MINERAL TANNING AGENT AND THEN IMPREGNATING THE RESULTING HIDES CONTAINING SAID MINERAL TANNING AGENT WITH AN AQUEOUS ALKALI METAL CYANATE SOLUTION U NTIL A QUANTITY OF ALKALI METAL CYANATE HAS BEEN ABSORBED SUFFICIENT TO SET THE MINERAL TANNING AGENT THEREIN AND CONVERT THE HIDES INTO LEATHER. 